This invention relates to a hydraulic axial piston pump supporting a rotating disk in a housing via hydrostatic bearings against a reaction force of pistons.
In an axial piston pump, a cylinder block has a plurality of pistons, each piston receives a reaction force in accordance with cylinder inner pressure and the reaction force is transferred to a rotating disk rotating along with the cylinder block. A force corresponding to the reaction of the pistons is exerted between a rear face of the rotating disk and the housing, and this force brings about large frictional force on sliding faces of the rotating disk and the housing.
It is known to provide hydrostatic bearings between sliding faces of the rotating disk and the housing in order to reduce the frictional force. The hydrostatic bearing is so constructed that a hole penetrating the piston is connected with a pocket provided at a sliding face of the rotating disk so as to conduct hydraulic pressure in the cylinder to the pocket. The hydraulic pressure in the pocket then acts between the rotating disk and the housing, to reduce contact pressure of the sliding faces and to reduce the frictional force therebetween.
The same number of pockets is provided as that of the pistons, and the inner pressure of each cylinder is conducted to a corresponding pocket. A half of one rotation of the cylinder block corresponds to a suction stroke with which the cylinder inner pressure becomes low and a remaining half of the rotation corresponds to a discharge stroke with which the cylinder inner pressure becomes high. The friction of the sliding faces is changed in accordance with the reaction force of the pistons and is large in the discharge stroke and small in the suction stroke. Therefore, the cylinder inner pressure conducted to the corresponding pocket via the through hole of the piston has a magnitude depending on the reaction force of the piston exerted on the rotating disk. Accordingly, large pressure is exerted in a region having large piston reaction force (discharge stroke) and small pressure is exerted in a region having small piston reaction force (suction stroke) to thereby maintain a balance for the hydrostatic bearing.
However, there is a case in which the high pressure is not exerted immediately to the pocket at a region of switching from the suction stroke to the discharge stroke. Although when the suction stroke is switched to discharge stroke in accordance with rotation of the cylinder block, the cylinder inner pressure is rapidly increased. However, there may be a small delay in transmitting this pressure change to the corresponding pocket. The delay depends on a volume of the pocket or the narrowness of a transmitting path.
In a transient period of time producing such a delay in response, there is a concern in which sufficient support force by the hydrostatic bearing is not produced, solid contact (metal contact) is brought about at the sliding faces and local wear or seizure of the sliding faces may be caused.
It is an object of this invention to resolve such a problem.
Specifically, it is an object of this invention to provide a pocket of a hydrostatic bearing with a sufficiently high pressure for preventing solid contact of the sliding faces in a region where switching from the suction stroke to the discharge stroke is performed.
A hydraulic pump according to this invention comprises a cylinder block rotatably supported in a housing, a plurality of cylinders arranged on a circle a center of which coincides with a center axis of the cylinder block, pistons respectively reciprocating in the cylinders, a suction port and a discharge port provided to the housing selectively communicating with the respective cylinders in accordance with a rotation position of the cylinder block, a drive shaft inclined relatively to the center axis of the cylinder block, a rotation transmitting mechanism for transmitting rotation of the drive shaft to the cylinder block so as to rotate the cylinder block synchronously with the drive shaft, a rotating disk rotating together with the drive shaft and cooperatively engaged with the pistons, a fixed sliding contact face formed in the housing, the fixed sliding contact face being brought into sliding contact with a rear face of the rotating disk, pressure pockets formed on the rear face of the rotating disk in correspondence with positions of the respective pistons, and pressure paths for conducting hydraulic pressure in the respective cylinders to the respective pressure pockets via the pistons.
The hydraulic pump further comprises a pressure introducing mechanism for introducing a high pressure fluid to a pocket moving along the fixed sliding contact face along with the rotating disk at a position in which a corresponding cylinder is about to switch from a suction stroke to a discharge stroke.
According to an aspect of this invention, the pressure introducing mechanism comprises a communication groove formed on a surface of the fixed sliding contact face for connecting the pocket at the position in which the corresponding cylinder is about to switch from the suction stroke to the discharge stroke, and an adjacent pocket corresponding to a cylinder in the discharge stroke.
It is preferable that an orifice is provided midway along the communication groove.
According to another aspect of this invention, the pressure introducing mechanism comprises a fluid path communicating with the discharge port and having an opening in the fixed sliding contact face to communicate with the pocket at the position in which the corresponding cylinder is about to switch from the suction stroke to the discharge stroke.
It is also preferable that the fluid path is provided with an orifice.
According to yet another aspect of this invention, the pressure introducing mechanism comprises a communication groove formed on the fixed sliding contact face and extending in a radial direction for communicating with the pocket at the position in which the corresponding cylinder is about to switch from the suction stroke to the discharge stroke, and a fluid path for introducing high pressure from the discharge port to the communication groove.
It is also preferable that the fluid path is provided with an orifice.
According to this invention, when the drive shaft is rotated, the cylinder block is rotated, the pistons are reciprocated in the respective cylinders, a working fluid is sucked from the suction port to expanding cylinders and the working fluid is discharged from contracting cylinders to the discharge port. Although a force corresponding to inner pressure of the contracting cylinders is exerted on the rotating disk as piston reaction force, the force is supported by a hydrostatic bearing constituted between the rotating disk and the fixed sliding contact face.
It is necessary that support force of respective pockets of the hydrostatic bearing is made to correspond to the force received from the corresponding pistons. Therefore, when the inner pressure of a cylinder is switched from suction pressure to discharge pressure, pressure of the corresponding pocket should be switched without delay.
The working fluid at high pressure is introduced to the pocket immediately before the cylinder is switched from the suction stroke to the discharge stroke by the pressure introducing mechanism. Therefore, the pressure of the pocket responses without being delayed when switching of the cylinder inner pressure takes place, and the hydrostatic bearing achieves always pertinent support force. As a result, excessively large frictional force is not produced between the rotating disk and the fixed sliding contact face, wear or seizure of the sliding faces does not occur, and the durability of the pump is enhanced.